Electronic ballast with pulsing output voltage

ABSTRACT

In a fluorescent lamp ballast, a source of high-frequency voltage is applied directly across a series-resonant L-C circuit. The fluorescent lamp is connected in parallel with the capacitor of the L-C circuit and a voltage-limiting means is operative to prevent the series-resonant L-C circuit from overloading the voltage source during any period when the lamp is not effective in providing circuit loading. When power is initially applied to the series-resonant L-C circuit, a control means provides a short circuit across the capacitor; and, by way of a first current transformer, the resulting short circuit current is used for pre-heating the fluorescent lamp cathodes. After about 1.5 second, the control means provides for removal of the short circuit for a period of about 25 milli-seconds, thereby permitting the voltage across the capacitor to grow to a magnitude sufficient to ignite and operate the lamp. If the lamp ignites, the resulting lamp current is then used, by way of a second current transformer, to prevent the control means from re-providing the short circuit. If lamp current does not flow, or if it at any time ceases to flow, the control means will re-provide the short circuit within about 25 milli-second. Thereafter, until power is removed or until an operable lamp is connected, the control means will continuously repeat the cycle of 1.5 second short circuit and 25 milli-seconds open circuit.

RELATED APPLICATIONS

This application is a Continuation of Ser. No. 07/793,280 filed Nov. 12,1991 now abandoned; which is a Continuation of Ser. No. 07/346,321 filedMay 1, 1989, now abandoned; which is a Continuation of Ser. No.06/686,275 filed Dec. 26, 1984, now abandoned; which is aContinuation-in-Part of Ser. No. 06/678,021 filed Dec. 4, 1984, nowabandoned; which is a Continuation-in-Part of Ser. No. 06/612,058 filedMay 18, 1984, now U.S. Pat. No. 4,667,131.

Application Ser. No. 07/793,280 is also a Continuation-in-Part of Ser.No. 07/513,053 filed Apr. 26, 1990, now U.S. Pat. No. 5,015,923; whichis a Continuation of Ser. No. 06/907,229 filed Sep. 15, 1986, nowabandoned.

Application Ser. No. 07/793,280 is also a Continuation-in-Part of Ser.No. 07/377,767 filed Jul. 10, 1989, now U.S. Pat. No. 4,928,039; whichis a Continuation of Ser. No. 06/678,021 filed Dec. 4, 1984, nowabandoned.

Application Ser. No. 07/346,321 is also a Continuation-in-Part of Ser.No. 07/145,925 filed Jan. 20, 1988 now abandoned.

Application Ser. No. 07/346,321 is also a Continuation-in-Part of Ser.No. 06/830,739 filed Feb. 19, 1986, now U.S. Pat. No. 4,893,059.

Application Ser. No. 07/346,321 is also a Continuation-in-Part of Ser.No. 06/830,270 filed Feb. 18, 1986, now U.S. Pat. No. 4,926,096.

Application Ser. No. 07/346,321 is also a Continuation-in-Part of Ser.No. 06/691,171 filed Jan. 14, 1985, now U.S. Pat. No. 4,644,228.

Application Ser. No. 06/686,275 is also a Continuation-in-Part of Ser.No. 06/612,058 filed May 18, 1984, now U.S. Pat. No. 4,667,131.

Application Ser. No. 06/686,275 is also a Continuation-in-Part of Ser.No. 06/495,540 filed May 17, 1983, now U.S. Pat. No. 4,554,487; which isa Continuation-in-Part of Ser. No. 06/411,263 filed Aug. 25, 1982, nowU.S. Pat. No. 4,461,980.

Application Ser. No. 06/686,275 is also a Continuation-in-Part of Ser.No. 06/677,562 filed Dec. 3, 1984, now U.S. Pat. No. 4,698,553; which isa Continuation-in-Part of Ser. No. 06/456,276 filed Feb. 22, 1983, nowU.S. Pat. No. 4,503,363.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to high-frequency series-resonant ballastsfor fluorescent lamps.

2. Prior Art

High-frequency series-resonant fluorescent lamp ballasts have beenpreviously described, such as in U.S. Pat. Nos. 3,710,177 to Ward and4,370,600 to Zansky. However, these previously described ballasts do notprovide solutions to several basic problems associated with practicalapplications of such ballasts. These problems relate to the excessivepower drain by and the self-destructive nature of the series-resonantballast under the condition of being connected to an inoperative lamp.

3. Background Considerations

In powering a fluorescent lamp by way of a high-frequencyseries-resonant ballast, where the ballast constitutes a high-Q resonantL-C circuit series-excited from an AC voltage source and parallel-loadedby the fluorescent lamp, there is a serious problem associated with thesituation where the fluorescent lamp for one reason or another ceases toconstitute an effective load for this high-Q series-excited L-C circuit.In such a situation, which is most apt to occur toward the end of normallamp life, the power drawn by the high-Q resonant L-C circuit from itsAC voltage source is so excessively high as to cause damage to or evendestruction of the L-C circuit and/or the AC voltage source. If,contrary to expectations, destruction of the circuit and/or the sourcedoes not occur, the amount of power drawn from the source will be solarge as to represent an unacceptable level of energy waste--recognizingthat it may often take a long time before a worn-out fluorescent lamp isreplaced.

SUMMARY OF THE INVENTION

Objects of the Invention

An object of the present invention is that of providing safe andefficient high-frequency series-resonant ballasts for fluorescent lamps.

This as well as other important objects and advantages of the presentinvention will become apparent from the following description.

Brief Description

An L-C circuit with an unloaded Q-factor of about 50 is series-connecteddirectly across the output of a 100 Volt/30 kHz voltage source. This L-Ccircuit is resonant at 30 kHz, which means that it is series-resonant atthe very frequency of the high-frequency voltage source.

A regular 40 Watt fluorescent lamp and a voltage-limiting means are bothconnected in parallel with the tank-capacitor of the L-C circuit, thevoltage-limiting means being operative to limit the voltage developedacross the tank-capacitor to a magnitude suitable for proper lampstarting, which magnitude is about 300 Volt.

Without the voltage-limiting means, with an unloaded Q-factor of 50 andlinear circuit operation, the magnitude of the voltage developing acrossthe tank-capacitor would have been 5000 Volt.

Under normal operation, the fluorescent lamp limits the magnitude of thevoltage developing across the tank-capacitor to about 100 Volt; and atthat point the loaded L-C circuit draws approximately 40 Watt of powerfrom the source. Thus, with a loaded Q-factor of about unity and anunloaded Q-factor of 50, the implication is that the losses in the L-Ccircuit amount to about 2% of the total power drawn from the sourceunder normal operating conditions.

If for some reason the fluorescent lamp should fail to constitute aneffective load for the L-C circuit, the magnitude of the voltage acrossthe tank-capacitor would increase to about 300 Volt, which implies thatthe power drawn from the source at that point would be about 120 Watt,with substantially all of it being dissipated in the voltage-limitingmeans.

If there were no voltage-limiting means present, however, the powerdrawn by the L-C circuit from the source--assuming no breakdown--wouldbe about 2000 Watt, with all of it being dissipated within the L-Ccircuit itself.

The present invention provides for means to prevent the L-C circuit fromoperating in its resonant mode--and thereby to prevent it from drawingexcessive power--in case the fluorescent lamp should fail even for abrief period to constitute a proper load for the L-C circuit. Thiseffect is accomplished by a transistor operative, by way of a rectifierbridge, to provide a short circuit across the tank-capacitor wheneverlamp current fails to flow for about 25 milli-seconds. With ashort-circuited tank-capacitor, the amount of power drawn by the L-Ccircuit is negligibly small.

More particularly, a control means is connected with the the L-C circuitand is operative to provide for the following functions.

a) Upon initially providing power to the L-C circuit, the control meansprovides for a short circuit across the tank-capacitor for an initialperiod of about 1.5 second; which is the length of time normallyrequired for the cathodes of the fluorescent lamp to become fullythermionic.

b) After this initial period, the control means removes the short ciruitfor a period of about 25 milli-seconds; which period is long enough toprovide for proper lamp starting under normal circumstances.

c) If lamp current starts to flow within this initial period, thecontrol means operates to keep the short circuit removed for as long aslamp current flows.

d) If lamp current fails to flow within this initial period, the controlmeans re-imposes the short circuit and keeps it so imposed for a periodof about 1.5 second; whereafter it again removes the short circuit for aperiod of about 25 milli-seconds.

e) It the fluorescent lamp is removed from the L-C circuit, or if forother reasons it fails to continue to operate, the control meansoperates to provide a short circuit across the tank-capacitor within aperiod of about 25 milli-seconds.

f) As long as there is no lamp loading the L-C circuit, the controlmeans tries every 1.5 second or so to start the lamp by removing theshort circuit for a period of about 25 milli-seconds. Thus, with aduty-cycle of about 25 milli-seconds out of 1500 milli-seconds (1.67%),the average power dissipation of the unloaded L-C circuit will be onlyabout 2.0 Watt.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 provides a schematic circuit diagram of the preferred embodimentof the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Details of Construction

FIG. 1 shows an AC voltage source S, which in reality is apower-line-operated frequency converter providing an output voltage of100 Volt RMS magnitude and 30 kHz frequency.

Connected directly across S is a series-combination of an inductor L anda capacitor C. Inductor L has a tightly coupled secondary winding Ls.

A Varistor V is connected directly across capacitor C.

A fluorescent lamp FL, having cathodes FLC1 and FLC2, is connected inseries with the primary winding CTp of control transformer CT, and thisseries-combination of FL and CTp is connected across capacitor C.

A bridge rectifier BR, having a B+ output bus terminal and a B- outputbus terminal, is connected in series with the primary winding Tp oftransformer T, and this series-combination of BR and Tp is connectedacross capacitor C.

A transistor Qa is connected with its collector to the B+ bus and withits emitter to the B- bus.

A series-combination of a capacitor C1 and a rectifier R1, with R1 beingconnected with the capacitor by way of its cathode, is connecteddirectly across the output of secondary winding Ls of inductor L. Thecathode of rectifier R1 is connected to the base of transistor Qa by wayof a resistor Ra.

A transistor Qb is connected with its collector and emitter to the baseand emitter, respectively, of transistor Qa.

Transformer T has a first secondary winding Ts1 connected with cathodeFLC1 of fluorescent lamp FL, and a second secondary winding Ts2connected with fluorescent lamp cathode FLC2.

Transformer T also has a tertiary winding Tt, which tertiary winding isconnected between the B- bus and the anode of a rectifier R2.

A series-combination of a resistor Rb and a capacitor C2 is connectedbetween the cathode of rectifier R2 and the B- bus, with one terminal ofresistor Rb being connected with the cathode of R2.

A series-combination of a resistor Rc and a Diac D is connected betweenthe base of transistor Qb and the junction between resistor Rb andcapacitor C2.

Control transformer CT has a secondary winding CTs connected between theB- bus and the anode of a rectifier R3. A rectifier R4 is connectedbetween the cathode of rectifier R3 and the base of transistor Qb, theanode of rectifier R4 being connected with the cathode of rectifier R3.

A capacitor C3 is connected between the cathode of rectifier R3 and theB- bus.

Details of Operation

In FIG. 1, when the 100 Volt/30 kHz voltage from source S is initiallyapplied to the L-C series-circuit, a voltage is developed acrosssecondary winding Ls. This voltage is rectified and filtered byrectifier R1 and capacitor C1, and then applied to the base oftransistor Qa by way of a current-limiting resistor Ra. The value of Rais so chosen that the resulting DC current provided to the base oftransistor Qa is adequate to make this transistor conduct in asubstantially saturated mode and thereby to constitute an effectiveshort circuit between the B+ bus and the B- bus. With transistor Qa soconducting, and since transformer T is a current transformer with a verylow-impedance primary winding, and since a short circuit at the outputof the bridge rectifier is essentially equivalent to a short circuit atits input, there is in effect a short circuit provided acrosstank-capacitor C. Thus, as long as transistor Qa is an effective shortcircuit, the magnitude of the current drawn from the source is limitedby the reactance of L, thereby in effect representing a non-dissipativeload.

As long as transistor Qa conducts, current is forced through the primarywinding Tp of transformer T. By transformer action, this current isprovided to the effective parallel connection of the two fluorescentlamp cathodes, thereby providing to these cathodes the modest amount ofpower required to bring about thermionic emission. The tertiary windingTt provides a voltage output that is used for charging capacitor C2 witha current that is limited by resistor Rb. Eventually, the magnitude ofthe voltage on C2 gets to be high enough to cause Diac D to break down,at which point the charge that had accumulated on capacitor C2 getsdischarged into the base of transistor Qb--with the magnitude of thedischarge current being principally determined by the resistance of Rc.This magnitude is so chosen that--as soon as the Diac breaksdown--transistor Qb becomes conductive to the point of shunting away thebase current provided to transistor Qa by way of resistor Ra.

In other words, as soon as the voltage on capacitor C2 has increased tosome pre-determined magnitude, the Diac breaks down and immediatelyrenders transistor Qa non-conductive. The time it takes for the voltageon capacitor C2 to reach this predetermined magnitude is a function ofthe time-constant associated with C2 and Rb as well as of the magnitudeof the voltage being provided by the tertiary winding Tt. In the circuitof FIG. 1, this time was chosen to be about 1.5 second; which is thelength of time normally required by fluorescent lamp cathodes to reachthe point of thermionic emission.

For as long as capacitor C2 is providing base current for transistor Qb,this transistor is operative to prevent current from being applied tothe base of transistor Qa, thereby making Qa non-conductive. The lengthof time during which Qa is thereby kept non-conductive is determined bythe parameters of capacitor C2 and resistor Rc. In the circuit of FIG.1, these parameters were so chosen as to make this length of time about25 milli-seconds.

Thus, after the initial period of about 1.5 second, during which Qarepresented a short circuit and the fluorescent lamp cathodes wereprovided with heating power, Qa is switched off and becomes an opencircuit for about 25 milli-seconds. During this 25 milli-second period,the voltage across capacitor C increases in magnitude to the point wherethe lamp starts. With already pre-heated cathodes, the time required forthe lamp to start is normally less than 25 milli-seconds.

As soon as lamp current starts to flow, control transformer CT, by wayof rectifier R3 and filter capacitor C3, provides a DC current to thebase of transistor Qb, thereby causing transistor Qb to continue toshunt away the base current for Qa. Thus, as long as the lamp starts todraw current within the 25 milli-second period, transistor Qa willcontinue to be non-conductive, and the lamp will continue to operate.

However, if lamp current does not start to flow within the 25milli-second period, base current for Qb will cease, which means that Qbwill cease shunting away the base current for Qa. Thus, after about 25milli-seconds, if lamp current fails to flow, transistor Qa will againbecome conductive and operative to provide a short circuit acrosscapacitor C.

Now, with Qa conductive, current will again flow through the primarywinding Tp of transformer P, and again will charge C2 to the point ofbreaking down the Diac; which then again starts a 25 milli-second periodof shunting away the base current for transistor Qa.

In other words, with the fluorescent lamp inoperative or disconnected,the circuit of FIG. 1 operates in a cyclical fashion, with each cycleconsisting of a 1.5 second period during which transistor Qa isconductive--which implies that capacitor C is shorted--and a 25milli-second period during which transistor Qa is non-conductive. Withthe lamp operating, on the other hand, this cyclical circuit operationis prevented by the flow of lamp current.

The lamp cathodes are supplied with heating power only as long as Qaconducts. After the lamp has ignited, however, heating power is nolonger needed and is no longer supplied, thereby providing for noticablyimproved lamp efficacy.

It is believed that the present invention and its several attendantadvantages and features will be understood from the preceedingdescription. However, without departing from the spirit of theinvention, changes may be made in its form and in the construction andinterrelationships of its component parts, the form herein presentedmerely representing the presently preferred embodiment.

I claim:
 1. An arrangement comprising:lamp output terminals operable toconnect with lamp input terminals of a fluorescent lamp; and an assemblyof interconnected component parts; the assembly being characterizedby:(a) including a power-line-operated frequency converter providing:(i) a first high-frequency output voltage at a first pair ofhigh-frequency output terminals; the first high-frequency output voltagehaving a fundamental period and a fundamental frequency; the fundamentalperiod being substantially shorter than that of the power line voltageon an ordinary electric utility power line; the fundamental frequencybeing substantially higher than that of the power line voltage on anordinary electric utility power line; and (ii) a second high-frequencyoutput voltage at a second pair of high-frequency output terminals; thesecond pair of high-frequency output terminals being connected incircuit with the first pair of high-frequency output terminals; (b)having its second high-frequency output terminals connected with thelamp output terminals; (c) whenever the lamp input terminals areconnected with the lamp output terminals, supplying a lamp current tothe fluorescent lamp; and (d) whenever the lamp input terminals are notconnected with the lamp output terminals, causing the secondhigh-frequency output voltage to exhibit periodic amplitude modulation,while the magnitude of the first high-frequency voltage remainssubstantially non-modulated; the periodic amplitude modulation having arelatively long period; the relatively long period being longer thanthat of the power line voltage.
 2. The arrangement of claim 1 whereinthe lamp output terminals are further characterized by: (i) including afirst and a second pair of lamp output terminals; and (ii) having anelectrically conductive path extending between the first pair of lampoutput terminals even when the lamp input terminals are not connectedtherewith.
 3. An arrangement comprising:a set of lamp output terminalsoperable to connect with a set of lamp input terminals of a fluorescentlamp; and a combination of constituent parts characterized by:(a)including a power-line-operated frequency converter providing: (i) afirst AC output voltage of relatively high frequency at a first set ofAC output terminals; the relatively high frequency being defined as afrequency substantially higher than that of the power line voltage on anordinary electric utility power line; the first AC output voltage havinga fundamental period substantially shorter than that of said power linevoltage; and (ii) a second AC output voltage at a second set of ACoutput terminals; the second set of AC output terminals being connectedin circuit with the first set of AC output terminals; (b) having thesecond set of AC output terminals connected in circuit with the lampoutput terminals; (c) at times when the lamp input terminals areconnected with the lamp output terminals, causing: (i) a lamp current tobe supplied to the fluorescent lamp, and (ii) the amplitude of thesecond AC output voltage to remain substantially constant; and (d) attimes when the lamp input terminals are not connected with the lampoutput terminals, causing the amplitude of the second AC output voltageto vary periodically at a relatively low frequency, even as theamplitude of the first AC voltage remains constant; the relatively lowfrequency being defined as a frequency lower than that of the power linevoltage.
 4. The arrangement of claim 3 wherein: (i) the set of lampinput terminals includes a first pair of lamp input terminals connectedwith a first thermionic cathode; (ii) the set of lamp output terminalsincludes a first pair of lamp output terminals operative to connect withthe first pair of lamp input terminals; and (iii) an electricallyconductive path exists between the first pair of lamp output terminalseven at times when they are not connected with the first pair of lampinput terminals.
 5. The arrangement of claim 3 wherein the combinationis further characterized in that the relatively low frequency has aperiod longer than one tenth of one second.
 6. An arrangementcomprising:a gas discharge lamp having a set of lamp input terminals;and a combination of constituent parts characterized by:(a) including apower-line-operated frequency converter providing: (i) a first AC outputvoltage of relatively high frequency at a first set of AC outputterminals, a relatively high frequency being defined as a frequencysubstantially higher than that of the power line voltage on an ordinaryelectric utility power line, the first AC output voltage having afundamental period substantially shorter than that of said power linevoltage; and (ii) a second AC output voltage of relatively highfrequency at a second set of AC output terminals, the second set of ACoutput terminals being connected with the first set of AC outputterminals by way of an impedance means; (b) having its second AC outputterminals connected in circuit with a set of lamp output terminals; thelamp output terminals being operable to connect with the lamp inputterminals; (c) at times when the lamp input terminals are connected withthe lamp output terminals, causing the amplitude of the second AC outputvoltage to remain substantially constant; and (d) causing the amplitudeof the second AC output voltage to vary periodically at a relatively lowfrequency, even as the amplitude of the first AC output voltage remainsconstant; the relatively low frequency being defined as a frequencysubstantially lower than that of said power line voltage.
 7. Anarrangement comprising:a gas discharge lamp having a set of lamp inputterminals; and a combination of constituent parts characterized by:(a)including a power-line-operated frequency converter providing: (i) afirst AC output voltage of relatively high frequency at a first set ofAC output terminals, a relatively high frequency being defined as afrequency substantially higher than that of the power line voltage on anordinary electric utility power line, the AC output voltage having afundamental period substantially shorter than that of said power linevoltage; and (ii) a second AC output voltage of relatively highfrequency at a second set of AC output terminals; (b) having its secondAC output terminals connected in circuit with a set of lamp outputterminals; the lamp output terminals being operable to connect with thelamp input terminals; and (c) with the lamp output terminals, causingthe amplitude of the second AC output voltage to vary periodically at arelatively low frequency, even as the amplitude of the first AC outputvoltage remains constant; the relatively low frequency being defined asa frequency substantially lower than that of said power line voltage. 8.The arrangement of claim 7 wherein the combination of constituent partsis further characterized by including an L-C circuit resonant at or nearthe fundamental frequency of the first AC output voltage.